Process of rolling alloy steel



Patented Dec. 23, 1930 UNITED STATES PATENT OFFICE HOWARD M. GIVEN S, OF ABINGTON, FRANCIS B. FOLEY, OF PHILADELPHIA, AND JOHN L. COX, OF SPRINGFIELD TOWNSHIP, MONTGOMERY COUNTY, PENNSYLVANIA PROCESS OF ROLLING ALLOY STEEL No Drawing.

Certain rollable and forgeable alloy steels, particularly those containing chromium, aluminum and silicon, with or without nickel and (or) tungsten or molybdenum, are par ticularly adapted for use in the manufacture of special articles, especially valves. In the manufacture of valves from such alloy steel, the metal is first rolled into bars, from which the finished articles are forged.

Great difliculty may be experienced in rolling the metal. Frequently the rolled. product is cracked or otherwise defective, necessitating scrapping and re-me-lting. Sometimes the rolled product is apparently sound but defects develop in the forging or in the finished article. It is obvious that if the problem of uniformly successful rolling could be overcome, it would be a decided contribution to the art.

We have discovered that the quality of the product made from such alloy valve steels and its freedom from surface defects can be greatly improved by a wide variation from the method customarily used in the rolling of steel. A further advantage of our new method of rolling is found in the fact that if of strongly martensitic type the steel bars resulting from such new method of rolling will be soft and readily shearable cold instead of being comparatively hard. The usual methods of rolling such steels frequently result'in the production of bars too hard and brittle to be cold sawn or sheared cold without cracking or fracturing, in such cases necessitating warming the bars in hot water or even in a furnace before shearing can safely be done. The expense attendant upon such manipulation is entirely avoided by our new method of rolling. If, however, the steel be 40 austenitic in structure, the same procedure cannot be followed to maximum advantage,

a wholly difierent procedure being preferable and forming the subject-matter of a separate application filed by one of the joint inventors of the process herein described and claimed. Methods of determining the structural character of steels are well known and no information is required to enable those skilled in the art to determine this factor. It may be said, however, that martensitic steel hardens Application filed November 20, 1929. Serial No. 408,671.

on exposure to the air after high heating, while austenitic steel does not. This affords one convenient method of determining structure.

If the alloy steel be stronglymartensitic in character, we have discovered that if it be rolled while within a certain high temperature range and be then allowed to cool througha wide range down to below its critical temperature of cooling (the Ar point) and be then reheated to a temperature somewhat close to, very preferably distinctly below, and certainly not above, its critical temperature on heat- 1ng (Ac point), the rolling may be resumed and completed while the alloy steel is cooling from thetemperature to which it has been reheated; the resultant product being free from defects. It is impossible to specify the precise temperature limits of the first rolling zone, or the precise temperature to which the alloy steel is reheated, since such temperatures vary somewhat with diiferent compositions. In most cases, but not in exceptional cases, the lower limit of the high temperature plastic Zone, in which the first rolling occurs, will be above the upper limit of the low temperature plastic zone, in which the second rolling occurs. The following example may be given. Given a composition comprising carbon 0.47%, chromium 10.20%, aluminum 1.85%, nickel 1.84%, silicon 0.92%, manganese 0.48% and the remainder principally iron, and assuming the steel to have been determined to be hard and martensitic on 85 cooling freely from the initial temperature of rolling, the alloy steel is first brought to a temperature of approximately 2300 F. or somewhat less and is rolled while cooling to a temperature not under about 2000 F. When the alloy steel has dropped to this temperature, the rolling, usually, will not have been completed. The alloy steel is then allowed to cool to below its critical temperature of cooling, that is, to about 1500 F. or less. 9! The alloy steel is then reheated to approximately 1900 F., whereupon rolling is resumed. The rolling is continued until'completion or until the metal has become too rigid to flow satisfactorily. The lower limit 1 of the second rolling temperature zone is relatively unimportant, but it is usually desirable to complete the rolling before the alloy steel has cooled much below 1400 F. or 1500 F.

The exact range of the first rolling temperature zone, which we call the high plastic temperature zone. and the approximate temperature to which the alloy steel should be elevated preparatory to rolling while the alloy steel is within the low plastic temperature zone, can be determined, in the case of any particular composition, by well known suitable tests. When these zones are ascertained, they may be safely assumed to apply to martensitic alloy steel of the same composition. As a factor of safety, the initial rolling may be carried out well within the limits of the ascertained high temperature plastic zone,

and the reheating may be carried to a temperature reasonably close to, but safely below, the upper limit of the low plastic temperature zone.

A further and great advantage gained by this method of rolling is that the resulting product is relatively soft and readily shearable. whereas bars rolled in the ordinary way, in addition to being of imperfect surface, may be so hard that they require annealing before they can be sheared or sawn.

Having now fully described our invention, what we claim and desire to protect by Letters Patent is:

1. The process of rolling alloy steels that are strongly martensitic in character which comprises rolling the alloy steel while it is cooling within a range whose upper limit is substantially above its critical temperature on heating and whose lower limit is substantially above its critical temperature on cooling, then cooling, without rolling, to a temperature below its critical temperature on cooling, then reheating to a temperature not above its critical temperature on heating, and

i then completing the rolling while it is cooling from the temperature to which it is reheated.

2. The process of rolling alloy steels that are strongly martensitic in character which comprises rolling the alloy steel while it is cooling within the range of its high plastic temperature zone, then allowing the metal to cool below its critical temperature on cooling, then reheating the metal well into a low plastic temperature zone whose upper limit is not above its critical range on heat ing, and completing the rolling while the metal is cooling within the low plastic temperature zone.

3. The process of rolling alloy steels that are strongly martensitic in character which comprises rolling the alloy steel while it is cooling within a range not below its critical temperature on heating, then cooling to below its critical temperature on cooling, then reheating to a temperature not above its said critical temperature on heating, and then completing the rolling while it is cooling from the temperature to which it is reheated.

4:. The process of rolling alloy steels that are strongly martensitic in character which comprises rolling the alloy steel while it is cooling within the range of its high plastic temperature zone, then allowing the alloy steel to cool to a relatively low temperature, then reheating the alloy steel to a temperature below the lower limit of the high plastic temperature zone, and completing the rolling while it is cooling from the temperature to which it is reheated.

5. The process of rolling alloy steels that are strongly martensitic in character which comprises rolling the alloy steel while it is cooling within a range above its critical temperature on heating, then cooling to below its critical temperature on cooling, then reheating to a temperature not above its said critical temperature on heating, and then completing the rolling while it is cooling from the temperature to which it is reheated.

6. The process of rolling alloy steels that are martensitic in character which comprises rolling the alloy steel while it is cooling within a temperature range of approximately 2300-2000 F., then allowing the metal to cool to a critical cooling temperature, then reheating the metal to a temperature of approximately l9 00 F., and then completing the rolling while the metal is cooling from the last named temperature.

In testimony of which invention, we have hereunto set our hands, at Philadelphia, Pennsylvania, on this 11th day of November,

HOWARD M. GIVENS. FRANCIS B. FOLEY. JOHN L. GOX. 

